Process for treating cellulosic materials with alkali and oxygen in the presence of complex magnesium salts

ABSTRACT

A PROCESS IS PROVIDED FOR TREATING CELLULOSIC METERIALS WITH ALKALI IN THE PRESENCE OF OXYGEN, AND PRTICULARLY AIR, AND IN THE PRESENCE OF COMPLEX MAGNESIUM SALTS OF AMINOPOLYCARBOXYLIC ACIDS OF ALKALI METAL SALTS THEREOF. THE INVENTION IS OF PARTICULAR APPLICATION TO THE REDUCTION OF LIGNIN CONTENT IN CELLULOSE PULPS AND THE DIGESTION OF WOOD CELLULOSE WITHOUT CAUSING DELETERIOUS DEGRADATION OF THE CELLULOSE, THE COMPLEX MAGNESIUM SALTS REDICING OR ENTIRELY PREVENTING ATTACK OF OXYGEN ON HEMICELLULOSE AND CELLULOSE CARBOHYDRATES, WITHOUT APPRECIABLY DIMINISHING THE OXIDATION OF THE LIGNIN AND ITS DISSOLUTION IN THE COURSE OF THE PROCESS.

United States Patent Oflice 3,701,712 Patented Oct. 31, 1972 3,701,712PROCESS FOR TREATING CELLULOSIC MATE- RIALS WITH ALKALI AND OXYGEN INTHE PRESENCE OF COMPLEX MAGNESIUM SALTS Hans Olof Samuelson, Goteborg,and Sture Erik Olof Norus, Ornskoldsvik, Sweden, assignors to M OchDomsjo Aktiebolag, Ornskoldsvik, Sweden No Drawing. Continuation-impartof applications Ser. No. 869,875, Oct. 27, 1969, now Patent No.3,652,386, and Ser. No. 36,670, May 12, 1970, now Patent No. 3,652,385.This application Feb. 26, 1971, Ser. No. 119,369 Claims priority,application Sweden, Mar. 2, 1970,

2,732/70 Int. Cl. D21c 9/10 US. Cl. 162-65 41 Claims ABSTRACT OF THEDISCLOSURE A process is provided for treating cellulosic materials withalkali in the presence of oxygen, and particularly air, and in thepresence of complex magnesium salts of aminopolycarboxylic acids oralkali metal salts thereof. The invention is of particular applicationto the reduction of lignin content in cellulose pulps and the digestionof wood cellulose without causing deleterious degradation of thecellulose, the complex magnesium salts reducing or entirely preventingattack of oxygen on hemicellulose and cellulose carbohydrates, withoutappreciably diminishing the oxidation of the lignin and its dissolutionin the course of the process.

This application is a continuation-in-part of Ser. No. 869,875, now Pat.No. 3,652,386, filed Oct. 27, 1969 and Ser. No. 36,670 now Pat. No.3,652,385, filed May 12, 1970.

It is known that chemical and semichemical cellulose pulps can betreated with oxygen in an alkaline medium in order to dissolve lignin.The oxygen treatment is carried out at an elevated temperature, of theorder of 100 C., and does not normally require more than one hour. Theamount of alkali required is of the order of 4 to 5% NaOH, based on thedry pulp. It is possible to obtain a good reduction in lignin content inthis way, but unfortunately, at the same time hemicellulose is alsodissolved, and a significant decomposition of the celluose takes place,as evidenced by a lower viscosity value. Also, the strength propertiesof paper manufactured from such treated pulp are poor.

It has been proposed (US. Pat. No. 3,384,533 to Robert et al., dated May21, 1968) that the process be improved by carrying out the treatment inthe presence of a metal carbonate, such as barium carbonate, calciumcarbonate, magnesium carbonate or zinc carbonate, in an amount withinthe range from about 0.5 to 3% by weight of the pulp. Of thesechemicals, magnesium carbonate gives the best results, when in an amountof approximately 1% by Weight of the pulp. However, magnesium carbonateis quite expensive, and the treatment is costly. Calcium carbonate,which is cheaper, is much less effective. In the case of all of thesesalts, the difficulty is that a powdered water-insoluble material mustbe charged to and mixed with the aqueous cellulose pulp system, and itis accordingly hard to obtain and maintain a homogeneous mixture, withuniform effect.

It has also been proposed (South African Pat. No. 3771/68) that inaddition to other water-insoluble magnesium compounds such as magnesiumoxide and hydroxide, or magnesium silicate, water-soluble magnesiumcompounds such as magnesium chloride and acetate could be used. These,however, react with the alkali present in the treating liquor to formthe water-insoluble hydroxide, and so do not actually resultin asolubilization of magnesium ion in the liquor.

In accordance with the present invention, a process is provided fortreating cellulosic materials with alkali in the presence of oxygen, andin the presence of a complex magnesium salt of an aminopolycarboxylicacid or alkali metal salt thereof, or a mixture of a magnesium salt andan aminopolycarboxylic acid or alkali metal salt thereof. In thisprocess, it has been found that it is possible in one stage to reducethe lignin content by more than 50% without causing deleteriousdegradation of the cellulose, or appreciable loss of hemicelluose. Infact, the dissolution of hemicellulose can be controlled so as to beinsignificant or appreciable, as desired, so that the process is alsoapplicable to hemicellulose dissolution. At the same time, thesecompounds are inexpensive, and since they are water-soluble, they can beadded in solution form, and form a homogeneous aqueous alkaline systemin which the cellulose pulp is suspended, and containing solubilizedmagnesium ion.

The process of the invention is particulary advantage ous in thealkaline treatment of lignin-containing wood cellulose in the presenceof oxygen gas or air, for the purpose of removing lignin. This processis referred to in the art as akaline oxygen gas bleaching. It is alsoapplicable to the controlled dissolution of hemicellulose in cellulosepulps, either during or after delignification, as well as to thealkaline digestion or pulping of wood in the presence of oxygen gas orair.

The terms treating and treatment as used herein are inclusive ofbleaching, digestion, and like processes, unless otherwise indicated.

The complex magnesium salts employed in the process of the inventionhave the important property of reducing or entirely preventing theattack of oxygen on the car bohydrates present in the cellulose andhemicellulose, without to any notably great extent affecting theoxidation of lignin and its dissolution. This protective effect is mostnoticeable with regard to the attack of oxygen on the cellulosemolecule, and primarily the attack of oxygen along the anhydroglucosechain of the cellulose molecule, an attack which gives rise to a rapidlowering of pulp viscosity. Thus, in the presence of the complexmagnesium compounds of the invention, the treated delignified pulp isfound to have a higher viscosity than would be obtained in theirabsence.

The process of the invention is applicable to unbleached, partiallybleached or bleached cellulose pulps, prepared from any cellulose sourceby any pulping process, for example, sulfate pulp, sulfite pulp andsemichemical pulp. The invention is especially applicable to cellulosepulps derived from wood, such as spruce pulp, pine pulp, hemlock pulp,birch pulp, fir pulp, maple pulp. alder pulp, aspen pulp, eucalyptuspulp, cherry pulp, sycamore pulp, hickory pulp, ash pulp, beech pulp,poplar pulp, oak pulp, and chestnut pulp. The invention is particularlyadvantageoiis in the preparation of any pulp in which it is especiallydesired to avoid degradation of the cellulose during processing, such asmost grades of paper pulp, and when it is desired to obtain a uniformcontrolled degradation, such as in the manufacture of viscose pulp of adesired viscosity.

'In most cases where the starting cellulose pulp is free of lignin, orwhere the lignin content is low, either naturally so, or because it hasben delignified, the process of the invention can be applied to removehemicellulose, and/or cause oxidation of end groups of the cellulose,with a regulated diminution of the pulp viscosity. In these processes,the complex magnesium compounds have the property of protecting thecellulose and hemicellulose molecules against uncontrolled degradation.

The oxygen digestion process of the invention is applicable to any kindof wood. In general, hardwood such as beech and oak can be pulped moreeasily than softwood, such as spruce and pine, but both types of woodcan be pulped satisfactorily using this process. Exemplary hard woodswhich can be pulped include birch, beech, poplar, cherry, sycamore,hickory, ash, oak, chestnut, aspen, maple, alder and eucalyptus.Exemplary softwoods include spruce, fir, pine, cedar, juniper andhemlock.

In the case of softwood, the processing conditions, including theparticle size of the wood fragments, the digestion temperature, thealkali concentration, and the oxygen pressure, should be carefullydetermined and controlled during the digestion.

The wood should be in particulate form. Wood chips having dimensionsthat are conventionally employed in the sulfate process can be used.However, appreciable advantages with respect to uniformity of thedigestion process under all kinds of reaction conditions within thestated ranges can be obtained if the wood is in the form of nonuniformfragments of the type of wood shavings or chips having an averagethickness of at most 3 mm., and preferably from about 0.2 to about 2 mm.Other dimensions are not critical. Sawdust, wood flour, slivers,splinters, wood granules and wood chucks and other types of foodfragments can also be used. In the case of softwood, the particlesshould be thin.

The complex magnesium aminopolycarboxylic acid salts in accordance withthe invention are formed from aminopolycarboxylic acids having theformula:

or alkali metal salts thereof, in which A is the group CH COOH or CH CHOH, where n is an integer from zero to five. The mono, di, tri, tetra,penta and higher alkali metal salts are useful, according to the numberof acid groups available and converted to alkali metal salt form.

Examples of such aminopolycarboxylic acids areethylenediaminetetraacetic acid, nitrilotriacetic acid,diethylenetriaminopentaacetic acid, ethylenediaminetriacetic acid,tetraethylenepentaamineheptaacetic acid, and hydroxyethylethylenediaminetriacetic acid, and their alkali metal salts, including the mono, di,tri, tetra and penta sodium, potassium and lithium salts thereof. Othertypes of aminocarboxylic acids which can be used to advantage areiminodiacetic acid, 2 hydroxyethyliminodiacetic acid,cyclohexanediaminetetraacetic acid, anthranil N,N diacetic acid, and2-picolylamine-N,N-diacetic acid.

These complexing acids can be present in rather large quantities, wellin excess of the amount of magnesium present, and within the range fromabout two to about ten times the amount needed to complex with,solubilize, and thus prevent precipitation of magnesium hydroxide orother insoluble magnesium salts during the treatment.

The magnesium complexes with these acids by forming salts with the acidgroups and by chelation with the nitrogen-containing groups or hydroxygroups, if present. Since the complexes are soluble in the alkalinetreating liquor, the precipitation of insoluble magnesium compounds iseifectively prevented.

It has been found that a particularly effective composition is acombination of the complex magnesium salts of the invention with othermagnesium compounds, and especially other complex magnesium compounds.

US. Pat. No. 3,652,386 discloses a class of watersoluble complexmagnesium compounds of magnesium and aliphatic alpha-hydroxycarboxylicacids of the type RCHOHCOOH and the corresponding beta-hydroxycarboxylicacids RCHOHCH COOH. These chelates are of the type:

In the above formula, n is zero or one. When n is zero, the acid is analpha-hydroxy acid, and when n is one, the acid is a beta-hydroxy acid.

R is the above formula is hydrogen oran aliphatic radical, which may bea hydrocarbon radical having from one to about ten carbon atoms, or ahydroxy-substituted hydrocarbon radical having from one to nine hydroxylgroups, and from one to about ten carbon atoms.

Exemplary alphaand beta-hydroxy carboxylic acids are glycolic acid,lactic acid, glyceric acid, 0a,,B-dihYdI'OXY- butyric acid,a-hydroxy-butyric acid, a-hydroxy-isobutyric acid, a-hydroxy-n-valericacid, a-hydroxy-isovaleric acid, B-hydroxy-butyric acid,fi-hydroxy-isobutyric acid, fl-hydroxy-n-valeric acid,fl-hydroxy-isovaleric acid, erythronic acid, threonic acid,trihydroxy-isobutyric acid, and sugar acids and aldonic acids, such asgluconic acid, galactonic acid, talonic acid, mannonic acid, arabonicacid, ribonic acid, xylonic acid, lyxonic acid, gulonic acid, idonicacid, altronic acid, allonic acid, ethenyl glycolic acid, andfi-hydroxy-isocrotonic acid.

Also useful are organic acids having two or more carboxylic groups, andno or from one to ten hydroxyl groups, such as oxalic acid, malonicacid, tartaric acid, malic acid, and citric acid, ethyl malonic acid,succinic acid, isosuccinic acid, glutaric acid, adipic acid, subericacid, azelaic acid, maleic acid, fumaric acid, glutanconic acid,citramalic acid, trihydroxy glutaric acid, tetrahydroxy adipic acid,dihydroxy maleic acid, mucic acid, mannosaccharic acid, idosaccharicacid, talomucic acid, tricarballylic acid, aconitic acid, and dihydroxytartaric acid.

The polyphosphoric acids are also good complexing agents for magnesium,and the magnesium salts of these acids are useful in combinations withthe complex magnesium aminopolycarboxylic acid salts. Exemplary aredisodium-magnesium pyrophosphate, trisodium-magnesium tripolyphosphateand magnesium polymetaphosphate.

Especially advantageous in such combinations from the standpoint of costare the acids naturally present in waste liquors obtained from thealkaline treatment of cellulosic materials. These acids represent thealkalior Watersoluble degradation products of polysaccharides which aredissolved in such liquors, as well as alkalior watersoluble degradationproducts of cellulose and hemicellulose. The chemical nature of thesedegradation products are complex, and they have not been fullyidentified. However, it is known that saccharinic and lactic acids arepresent in such liquors, and that other hydroxy acids are also present.The presence of C -isosaccharinic and C metasaccharinic acids has beendemonstrated, as well as C; and C -metasaccharinic acids. Glycolic acidand lactic acid are also probable degradation products derived from thehemicelluloses, together with beta-gamma-dihydroxy butyric acid.

Carbohydrate acid-containing cellulose waste liquors which can be usedinclude the liquors obtained from the hot alkali treatment of cellulose,liquors from sulfite digestion processes, and liquors from sulfatedigestion processes, i.e., kraft waste liquor. The waste liquorsobtained in alkaline oxygen gas bleaching or digestion processesandalkaline peroxide bleaching processes can also be used. lln thisinstance, the alkaline liquor can be taken out from the processsubsequent to completing the oxygen gas treatment stage, or during theactual treatment process.

The complex magnesium salts can be formed first, and then added to thecellulose pulp. They can also be formed in situ from a water-soluble orwater-insoluble magnesium salt, oxide or hydroxide, in admixture withthe complexing acid, the aminopolycarboxylic acid, hydroxycarboxylicacid, or polyphosphoric acid, or salt thereof, and this mixture can beadded to the pulp. For instance, a waste liquor employed as a source ofcomplexing acid or anhydride or salt thereof can be mixed with amagnesium salt, oxide or hydroxide, before being introduced to theprocess, or the magnesium salt, oxide or hydroxide can be added to thepulp, and then the pulp brought into contact with the complexing acid oranhydride or salt thereof. It is also possible to combine the complexingacid or anhydride or salt thereof with the pulp, and then add themagnesium salt, oxide or hydroxide, but this method may be lessadvantageous in practice.

In whatever form the magnesium is added, whether as salt, oxide,hydroxide, or complex salt, the amount of magnesium is calculated asMgO.

A noticeable improvement in the treating process is obtained when aslittle magnesium as 0.005% MgO, calculated on the dry weight of thepulp, is added. A high proportion of magnesium, up to 1% MgO, calculatedon the dry weight of the pulp, has been employed without disadvantageouseifect. However, for economic reasons, it is usually desirable to use aslittle magnesium as possible, and normally an amount within the rangefrom about 0.01 to about 0.5% MgO, calculated on the dry weight of thepulp, is employed.

Upon conclusion of the alkaline oxygen gas treatment, it is possible toseparate the magnesium-containing waste liquor and recycle it for reuse.The consumption of magnesium salts is negligible, and usually it is noteven necessary to replenish the magnesium content before recycling.However, additional magnesium compound can be added before recycling, ifnecessary, to restore the magnesium content, as MgO, and maintain a highenough level, for instance, to prevent oXidative degradation of thecellulose or hemicellulose. The consumption of magnesium salt has beennoted to be particularly low when waste liquor from a part of thealkaline oxygen gas treatment process is employed as the source ofcomplexing acid, and recycled for continued treatment of new batches ofpulp.

Some wood pulps are particularly high in magnesium ion because of thenature of the pulp or of the pulping process. For example, unbleachedpulps produced by digestion of wood with magnesium bisulfite ormagnesium sulfite usually contain enough magnesium ion so that noaddition of magnesium compound need be made. Waste liquors from theseprocesses can be used per se, in the process of the invention, inasmuchas they already contain the complexing acids, and a suflicientproportion of magnesium ion as well.

As a source of magnesium, one may add any magnesium salts, oxide orhydroxide, either to regenerate a spent treatment liquor, or to preparea waste liquor 'or other material for use in the process. Anywater-soluble or -insoluble magnesium compound can be used, such as, forexample, magnesium sulfate, magnesium chloride, magnesium bromide,magnesium chlorate, magnesium potassium chloride, magnesium formate,magnesium acetate, magnesium oxide, magnesium hydroxide, and magnesiumnitrate. If it is desired to recover the liquor after the treatment,then it is usually preferable to employ magnesium sulfate, so as toavoid the introduction of foreign anions into the system. Magnesiumcompounds which have no deleterious anion or which have an anion whichis destroyed in the course of the process, such as magnesium oxide,magnesium hydroxide, and magnesium carbonate, are also advantageous.Since these are waterinsoluble, it is desirable, however, to combinethese with the complexing agent in the presence of water, and awaittheir dissolution, indicating that the complex has been formed, beforecombining with the pulp, or before commencing the alkaline oxygen gasreaction. Any other water-insoluble magnesium compounds can be used inthis way, for instance, magnesium oxide or hydroxide, magnesiumphosphate, magnesium silicate and magnesium sulfide.

The alkaline treatment of the pulp in the presence of oxygen is carriedout in the normal way. Since the processes differ somewhat in the stepsand conditions, bleaching (with delignification) and digestion (withdelignification) are considered separately, in that order.

In bleaching, in order to obtain a rapid reaction between the cellulosicmaterial and the oxygen gas or air supplied to the system, the partialpressure of oxygen at the beginning of the bleaching should be at leastone atmosphere. However, lower pressures can be used, when a slowerreaction is acceptable. When using pure oxygen gas, the process can becarried out at pressures approximating atmospheric pressure, while ifair is used, because of the lower proportion of oxygen, higherpressures, usually superatmospheric pressures, are employed. If oxygenis used, a practical upper limit is 20 atmospheres, while if air isused, a practical upper limit is 60 atmospheres. The higher thepressure, the more rapid the reaction. Usually, an oxygen gas pressurewithin the range from about 2 to about 12 atmospheres is preferred.

It is frequently expedient to supply the oxygen gas or air during theprocess, and to release air enriched with regard to inert gas during theprocess.

The reaction will proceed at low temperatures, of the order of 25 to 50C., but then the reaction is slow, and a large reaction vessel isnecessary. Consequently, in order to reduce reaction time to a practicalrange, and keep the equipment small, the bleaching is usually carriedout at a temperature within the range from about to about 150 C. If itis desired to reduce the viscosity of the pulp, the higher temperaturescan be used, of the order of 130 to 140 C. When treating sulfate paperpulps, a lower temperature is used, if a significant reduction of thehemicellulose content is not desired. If a significant reduction of thehemicellulose is desired, however, then it is desirable to employ arather high temperature. Usually, in the case of sulfate paper pulps,the treatment is carried out advantageously at from to C.

The temperature can be varied upwardly or downwardly, progressively orcontinuously, during the process. It is in many cases desirable to beginthe reaction at a low temperature, and then to gradually increase thetemperature during the reaction. This is particularly true in the caseof pulps containing hemicellulose which in an unoxidized condition isattacked by alkali, for example, sulfite pulps, and semichemical pulps.Thus, the reaction temperature is low while the hemicellulose remainsunoxidized, but as it becomes oxidized, in the course of the reaction,the temperature can be increased, thus reducing the total reaction time.

The concentration of cellulosic material in the reaction mixture can bevaried within wide limits, and is in no way critical. Concentrationswithin the range from about 3 to about 45% are employed. It is, however,preferable to effect the treatment at a concentration in excess of 10%,and preferably within the range from about 15 to about 35%. When highpulp concentrations are treated, the pulp should be shreddedmechanically after or at the same time as the reagent chemicals areadded to the reaction mixture.

In a preferred embodiment of the invention, which gives a particularlyuniform bleaching and a pulp whose properties can be controlled withinthe narrow limits, the cellulosic material is first impregnated with anaqueous solution of the complex magnesium salt, or an aqueous solutionof the components which in admixture give rise to the complex magnesiumsalt, before being treated with air or oxygen. The excess of theimpregnating solution can then be removed, for example, by filteringand/or by pressing, before the treatment is begun. The solution that isremoved can, of course, be used for impregnating additional cellulosicmaterial.

The amount of alkali required in the bleaching depends on the quantityof lignin and/or hemicellulose which it is desired to remove. Normally,the alkali charge (calculated as NaOH) is within the range from about0.5 to about 12% NaOH, based on the weight of the cellulosic materialpresent. Other alkalis can be used, such as potassium hydroxide orlithium hydroxide, and sodium carbonate, in which event the amounts arechanged proportionately. If it is desired to dissolve large quantitiesof lignin and/or hemicellulose during the process, an alkaline chargewithin the range of about 7 to about 12% can be used. When bleaching apulp having a low lignin content, in which case a smaller amount oflignin and/or hemicellulose is to be dissolved, the charge can be withinthe range from about 0.5 to about 7%.

The proportion of hemicellulose dissolved decreases as the amount ofalkali is reduced, and accordingly, the amount of both the lignin andthe hemicellulose dissolved can be regulated by control of the amount ofalkali added.

It may be advantageous to add only a portion of the total quantity ofalkali at the beginning of the process, and then add additional alkalias the reaction proceeds. The alkali attacks the lignin preferentially,and by limiting the amount of alkali present at any given time, it ispossible to remove the lignin with a minimum of attack upon thecellulose and hemicellulose in the course of the reaction. The desiredgrade of pulp can thus be controlled by the manner and rate at which thealkali is charged to the system, and the size of the alkali charge, andthe reaction time.

The alkali can be combined with the pulp either before, during, or aftercombination with the complex magnesium salt, and it can be introduced inwhole or in part in this way. The mixing with alkali can be effected atthe desired reaction temperature, or at a lower temperature, after whichthe temperature is increased to reaction temperature.

The reaction time required depends upon the oxygen gas pressure and thereaction temperature. If the oxygen gas pressure is high, and thereaction temperature is high, the reaction can be complete in rather ashort time, for example, five minutes. When oxygen gas is employed atatmospheric pressure, reaction times of ten hours and more can be used.Normally, however, in a commercial process, where a high rate ofproduction per hour is desirable, the reaction times will be Within therange from about 10 to about 120 minutes. The reaction time is easy tocontrol, since the reaction halts when the alkali is consumed, and thusthe reaction time can be increased or shortened, depending upon theamount of alkali added at any given time, for a given gas pressure andtemperature of reaction.

The bleached and delignified pulp can be further processed in accordancewith known methods, as desired. It can, for example, be bleached withchlorine and/ or sodium chlorate and/or chlorine dioxide, and it mayalso be subjected to continued refinements, in accordance with knownprocedures.

The digestion process of the invention under controlled conditions andmoderate oxygen pressures digests wood with a mixture of alkali andoxygen and obtains in high yield a cellulose pulp having a highbrightness and a low lignin content. What is essential in order toobtain these results is to limit the amount of alkali at the beginningof the digestion to at most 75%, and preferably from about to about 20%,of the total molar quantity of alkali required for the digestion, and toadd the alkali progressively, either continuously or in increments,during the digestion, while maintaining the pH of the digestion liquorin the course of the digestion Within the range from about 9.2 to about13, and preferably from about 9.5 to about 11.5.

The total amount of alkali that is required for the digestion isdetermined by the quality and type of the pulp to be produced, and iswithin the range from about 1 to about kilomoles per 1000 kg. of drywood. It is well known that certain types of pulp are more digested thanothers. This is entirely conventional, and does not form a part of theinstant invention. Cellulose pulps intended to be used in the productionof regenerated cellulose fibers, such as viscose, acetate andcuprammonium pulps, are quite fully digested, and should have a lowcontent of lignin and hemicellulose. In the production of such pulps, inaccordance with the process of the invention, the amount of alkali canbe Within the range from about 6 to about 8 kilomoles per 1000 kg. ofdry wood. Semichemical pulps are given an intensive mechanical treatmentfollowing their digestion, in order to liberate the cellulose fibers,and in the production of such pulps, using the process of the invention,the amount of alkali can be much less, within the range from about 1 toabout 2 kilomoles per 1000 kg. of dry wood. For the production of brightpaper pulp, which is readily defibered when the digester is blown, theamount of alkali used in the process of the invention can be Within therange from about 2.5 to about 5 kilomoles. Generally, for most of thetypes of pulps given an intermediate degree of digestion, such as pulpsfor fine paper, plastic fillers, and soft paper or tissue paper, theamount of alkali in the process of the invention is within the rangefrom about 2 to about 6 kilomoles per 1000 kg. of dry wood.

Any alkali metal hydroxide or alkali metal carbonate can be employed,such as sodium hydroxide, potassium hydroxide, lithium hydroxide, sodiumcarbonate, potassium carbonate and lithium carbonate. The sodiumcarbonate obtained in the burning of cellulose digestion waste liquorscan be used for this purpose. The use of alkali metal carbonates may bemore advantageous than the use of alkali metal hydroxides in maintainingthe pH of the digestion liquor within the stated range, because of thebuffering properties of the carbonate or bicarbonate prescut or formedin situ. Consequently, mixtures of alkali metal hydroxides and alkalimetal carbonates are particularly satisfactory, to obtain the advantagesof each, and dilute their disadvantages.

It is also possible to use mixtures with alkali metal hydroxides orcarbonates with alkali metal bicarbonates, such as sodium bicarbonateand potassium bicarbonate. The alkali metal bicarbonate in this caseserves as a bufler. Other buffering agents of alkali metals withnondeleterious acidic anions can be employed, such as alkali metal acidphosphates, and alkali metal acid or bisulfites, such as potassiumdihydrogen phosphate, potassium monohydrogen phosphate, sodiumdihydrogen phosphate, sodium monohydrogen phosphate, sodium acidsulfite, and potassium acid sulfite, as well as the lithium salts ofthese anions.

The amount of bulfering agent such as alkali metal bicarbonate isusually within the range from about 1 to about 5 kilomoles per 1000 kg.of dry wood. The alkali metal bicarbonate or other buffering agentshould be added to the digestion liquor either initially or at an earlystage of the digestion. The addition of the bicarbonate or otherbuffering agent increases the buffer capacity of the digestion liquor,thereby assisting in avoiding variations in pH outside the prescribedrange during the digestion. The buffering agent, particularly abicarbonate, is especially desirable when it is desired to operate at arelatively low pH, for example, from about 9.2 to about 9.5. In thiscase, bicarbonate or other buffering agent can be added to advantageeven if alkali metal carbonate is present.

Large amounts of buffering agents, and particularly bicarbonates, shouldbe avoided, however, since the presence of large amounts of additionalforeign anions can be undesirable. In the case of bicarbonates, carbondioxide may be produced in the course of the digestion as the buifer isconsumed. The carbon dioxide dilutes the oxygen, and adds an extra loadto the chemical recovery system, and is therefore undesirable in largeamounts. However, the addition of minor amounts of the buffering agentwithin the stated range contributes to pulp uniformity because of itsassistance in maintaining pH.

It is also important to restrict the amount of bicarbonate or otherbuffering agents to an amount which is sol uble in the digestion liquor,so as to avoid precipitation thereof Within the Wood being digested.

Also useful as a buffer are the base liquors from previous digestionsand/or the waste liquors from oxygen bleaching processes, such as thosedescribed in US. Pats. Nos. 3,652,385 and 3,652,386 referred to above.In this way, better economy is obtained in chemical recovery, which canbe effected after evaporating and burning the waste digestion liquor,using known methods.

For economic reasons, the sodium compounds are preferred as the alkalimetal hydroxide, alkali metal carbonate and alkali metal bicarbonate.

It is also possible to add the additional chemicals normally present indigestion liquors, such as sodium sulfate as well as small amounts ofsodium sulfide or other alkali metal sulfide. At most, such chemicalsare added in an amount of about 1 kilomole per 1000 kg. of dry Wood.

Limiting the amount of alkali metal hydroxide and/ or alkali metalcarbonate in the initial stages of the process is quite important inobtaining a cellulose pulp of the desired quality. At most, 75% of thetotal molar quantity required of the alkali can be added ab initio, andeven this high percentage is only desirable if the pulp to bemanufactured is a semichemical pulp, or if the wood has been pretreatedwith sulfur dioxide in aqueous solution. For most pulps, including eventhe semichemical pulps, a better cellulose pulp is obtained if theinitial charge of alkali is within the range from about 2 to about 50%of the total molar quantity required for the digestion. The remainder ofthe alkali is added progressively, either incrementally or continuously,as the digestion continues. When producing bright pulps having a lowlignin content, it is satisfactory to charge not more than 20% andsuitably from about 5 to about 20% of the alkali at the beginning of thedigestion process.

If a mixture of alkali metal hydroxide and alkali metal carbonate isused, it is particularly suitable if the initial charge comprises sodiumcarbonate optionally with an addition of sodium bicarbonate as describedabove, the remainder of the alkali added as the digestion proceeds beingsodium hydroxide. If the alkali charge initially is alkali metalhydroxide, it is usually important in producing pulps having a lowlignin content that the initial charge be low, within the range fromabout 2 to about of the total molar quantity of alkali.

Whether or not the digestion process is carried out continuously or as abatch process, the alkali metal hydroxide and/or alkali metal carbonatecan be charged continuously or in increments to the digestion liquor. Ina continuous digestion, the wood is caused to move through the digesterfrom one end to the other which thereby constitutes a reaction zone. Ina batch process, the wood usually in the form of chips is retained inthe reaction vessel throughout the digestion.

Since the oxygen that is employed as an essential component in thedigestion process of the invention is a gas, the so-called gas phasedigestion procedure can be used to advantage. In this case, the wood andthe film of digestion liquor present on the wood are kept in continuouscontact with the oxygen-containing gas. If the wood is completely orsubstantially immersed in the digestion liquor, it is important toagitate the wood and/ or the gas and/or atomize the gas or the liquor.The oxygen should be dissolved or dispersed in the digestion liquor tothe greatest extent possible. Dissolution or dispersion of the oxygen inthe liquor can take place within the digestion vessel and/or externallyof the same, such as in nozzles, containers or other known devices usedfor dissolving or dispersing gases in liquids.

Transfer of oxygen to the wood material impregnated with digestionliquor is important in the process, and is controlled by adjusting theoxygen pressure, the digestion temperature and/or the proportion ofgas-liquid contact surfaces, including the wood impregnated withdigestion liquor.

The oxygen is preferably employed as pure oxygen, but mixtures of oxygenwith other inert gases can be used, such as, for example, mixtures ofoxygen with nitrogen and with carbon dioxide and with both, as well asair. Compressed air can also be used, although this complicates thedevices for dissolving or dispersing the oxygen in the reaction mixture.

Prior to contact with the oxygen, the wood suitably in the form of chipscan be impregnated with an aqueous digestion liquor containing thedesired chemicals. The chips are impregnated under vacuum, or underatmospheric pressure or superatmospheric pressure, or by other methodsconventional in wood digestion processes. The wood may also be treatedwith steam before being brought to the digestion zone.

The temperature employed during the impregnation can be within the rangefrom about 20 to about 120 C., although temperatures within the rangefrom to C. would not normally be used except under specialcircumstances. In the latter case, the highest temperature during thedigestion may be the same as the impregnating temperature, as well asthe initial digestion temperature. Generally, however, it is toadvantage if the digestion temperature is allowed to rise during thedigestion process, so that normally the temperature during impregnationwould not exceed about 60 C.

The digestion can be carried out at a temperature within the range fromabout 60 to about 175 C. Usually, it is advantageous if the digestiontemperature is permitted to rise during the digestion process from aninitial temperature of the order of from 60 to 90 C. to the maximumdigestion temperature of the order of from 110 to or C.

At a maximum temperature of 90 C., the digestion process proceedsslowly, but on the other hand, moderate oxygen pressure and simpletechnical apparatus can be used. A digestion temperature of from 90 to110 C. can be used to advantage when producing semi-chemical pulps, thefibers of which are not fully liberated until after subjection to amechanical treatment process, such as in a rlefiner, after the digestionprocess. These are high yield pu ps.

If a maximum digestion temperature of from 150 to 175 C. is used, thedigestion will proceed rapidly. On the other hand, in this case anexceedingly effective transfer of oxygen to the wood from the gas phaseis required. Thls requires intimate contact and high oxygen pressure. Byeffective control methods, however, all of which are conventional, it ispossible to control the digestion within this temperature range,particularly when producing cellulose pulp of moderate yield.

Normally, a maximum digestion temperature within the range from 110 to150 C. is preferred at which temperature the digestion can take place ina reasonable time using relatively simple apparatus and under moderateoxygen pressure, with good control of pulp quality, irrespective ofwhether semichemical pulps are being produced or cellulose pulps whosefibers can be liberated without intensive mechanical treatment, or aresimply liberated when the cooker or digestion vessel is blown.

The partial pressure of oxygen during the digestion process should bewithin the range from about 1 to about 20 atmospheres, preferably fromabout 3 to about 20 atmospheres. Higher pressures should not be used,from the standpoint of safety, and are definitely unnecessary. At lowerpressures, the digestion proceeds more slowly, and such pressures arenot economically practical. Normally, a pressure within the range fromabout 3 to about 12 atmospheres is preferred.

Because of the consumption of oxygen in the course of the digestion, andthe higher rate at which the digestion proceeds at high reactiontemperatures, it follows that the higher the reaction temperature, thehigher the pressure that should be applied during the reaction. Theoptimum temperature and pressure conditions for a given pulp can bedetermined by digestion sampling procedures, as is well known. Suchtrial-and-error experimentation is conventional, and is not a part ofthis invention.

It is often suitable during the digestion to withdraw a portion of thedigestion liquor, such as by draining, pressing, displacement orfiltering. This liquor can be returned to the digestion process at alater stage, or to a subsequent batch, and in this event it isadvantageous to heat the liquid or a part thereof under pressure to anelevated temperature of the order of from 110 to about 200 C. inintimate contact with an oxygen-containing gas such as air in order tooxidize organic substances in the liquor. The liquor can be fortified byadding additional alkali metal hydroxide and/or alkali metal carbonateand/or inhibitor before or after pressure-heating.

In the manufacture of many types of cellulose pulp, particularly viscosepulp, cuprammonium pulp, and paper pulp having a high degree of opacity,it has been found advantageous to pretreat the wood before the digestionwith water or an aqueous acidic, neutral, or alkaline solution,optionally in several stages. In the case of such pulps, thispretreatment is preferably effected at a high temperature, within therange from about 90 to about 150 C., whereupon a marked dissolution offrom about 2 to about 15% of the wood material takes place. Waste liquorfrom wood cellulose alkaline treatment processes or alkaline digestionprocesses can be used as the alkaline medium, and the treatment may becontinued until the pH of the solution drops and the solution becomesacidic. It has been found particularly suitable to use in thispretreatment the alkaline waste liquor, removed subsequent to or duringthe oxygen digestion process of the instant invention.

When producing high strength paper pulps, the pretreatment can becarried out with such solutions at lower temperatures, from about 30 toabout 100 C., preferably using an acid solution such as a 0.1 to 1%aqueous solution of sulfuric acid, nitric acid, or phosphoric acid.These acids can also be used in high temperature pretreatment.

Before carrying out the oxygen digestion process of the presentinvention, it is particularly suitable to pretreat the wood with anaqueous solution containing-sulfur dioxide, sodium bisulfite and/orsodium sulfite or other alkali metal sulfite such as potassium bisulfiteor sulfite. The treatment causes some dissolution and modification ofthe wood material, which has been found to be favorable during theoxygen digestion, particularly in the case of wood material which isdiflicult to pulp without any form of pretreatment, such as softwood. Bypretreating the wood in this manner with water or aqueous solutions, thepulp can be modified to any desired degree, and by suitably selectingthe conditions according to trial-anderror experimentation (which can becarried out on a small sample) to suit the wood used, the treatingconditions can be optimized for different fields of use of the pulpproduct. Generally speaking, the pretreatment causes a reduction in theconsumption of alkali during the oxygen digestion process of theinvention.

When producing many types of pulps which should be metal-free, orsubstantially so, such as viscose and cuprammonium pulps and pulps forhigh strength papers, the pretreatment stage or part thereof ispreferably carried out in the presence of a complexing agent forbivalent and/or polyvalent metal ions, such as copper, iron, manganese,cobalt and vanadium. In this way, it is possible to remove and/or renderharmless ions of the so-called transition metals, which catalyze anoxidative degradation of the carbohydrates during the subsequentdigestion process. Examples of suitable complexing agents are chelatingsalts of nitrogen-containing polycarboxylic acids of the class set forthabove in conjunction with the magnesium complex as well aspolyphosphates and ethylenediamine and ethylenediamine derivatives,although other complexing agents of an inorganic or organic nature canalso be used to advantage. The effect can be increased if mixtures ofdifferent complexing agents are used, since certain complexing agentshave more of an affinity for certain polyvalent metal ions than others,and a blend is better capable of chelating a mixture of polyvalent metalions for this reason. The use of complexing agents in connection withthe pretreatment has been found to promote uniformity of the pulp duringdigestion.

It may be desirable to wash the wood with water between the pretreatmentstage and the oxygen digestion process. This washing step may bedesirable in the case of any of the pretreatment processes describedabove. The washing, however, increases the cost of the processing, andalso increases the risk of water contamination of the pulp with metalions and metal compounds, and consequently it may often be morepractical to omit the washing step, unless it can be carried out withdeionized water, at 'low cost. Omission of the washing is usuallydisadvantageous.

It has also been found advantageous to have complexing agents forbivalent and/or polyvalent metal ions present during the oxygendigestion process. Any complexing agents which are stable and notdeleteriously affected by the digestion liquor can be used. Suitablecomplexing agents include those mentioned above.

After the oxygen digestion process has been completed, the pulped woodmay optionally be subjected to a mechanical treatment in order toliberate the fibers. If the pulping is brief or moderate, a defibrator,or disintegrator, or shredder, may be appropriate. After an extensive ormore complete pulping or digestion, the wood can be defibrated in thesame manner as in other conventional cellulose cooking processes, suchas sulfate pulping, by blowing off the material from the digester, or bypumping.

The pulped wood cellulose that is obtained in accordance with theprocess of the invention is of such whiteness that it can be used toadvantage directly for producing tissue paper, light cardboard andmagazine paper. When a higher degree of brightness is desired as forfine paper, rayon and cellulose derivatives, the pulp can easily bebleached in accordance with known methods by treatment with chlorine,chlorine dioxide, chlorite, hypochlorite, peroxide, peracetate, oxygenor any combinations of these bleaching agents in one or more bleachingsequences as described in for example US. Pat. No. 3,652,388. Chlorinedioxide has been found to be a particularly suitable bleaching agent forthe oxygen digested cellulose pulp obtained in accordance with thisinvention. The consumption of bleaching chemicals is generally markedlylower in bleaching oxygen digested pulps of the invention than whenbleaching sulfate cellulose.

The chemicals used for the digestion process can be recovered after thewaste liquor is burned and subsequent to optionally causticizing all orpart of the carbonate obtained when burning the liquor.

Preferred embodiments of the digestion process of the invention and ofthe cellulose pulps of the invention are shown in the followingexamples:

EXAMPLE 1 Unbleached pine sulfite pulp having a kappa number of 33.9 anda viscosity of 1181 cm. g. according to SCAN cp. according to TAPPI) wasbleached and delignitied in the manner described under (a) to (e) below.The following general procedure was used in all runs.

The pulp was finely divided in a peg shredder at a dry content of 30%.Water and sodium hydroxide were added in amounts to give a 3% pulpconcentration. The pulp suspension was then vigorously stirred with apropeller agitator, and the additives mentioned below were blended in.The pulp was then separated by filtration, and squeezed to a dry solidscontent of 27%, after which the pulp was shredded in a peg shredder. Theoxygen gas bleaching was carried out under an oxygen gas pressure of 8kg./ :m.

13 at 100 C. for 15 minutes. The pulp was then removed and washed withwater.

(a) This pulp was bleached as a control, with no other additives thanwater and NaOH, and contained 3.2% NaOH based on the dry pulp during theoxygen gas treatment stage.

(b) This pulp was bleached using complex magnesium salts as an additive,obtained by mixing magnesium sulfate in waste liquor pressed from pulptreated with oxygen gas according to (a), the hydroxy carboxylic acidsin the waste liquor serving as complexing agents. The waste liquor waspressed from the pulp before the pulp was washed. The waste liquorcontaining magnesium complexes replaced of the added water in (a), andwas mixed together with water and sodium hydroxide into the pulp, afterwhich the bleaching was carried out in the same manner as that of (a).The pulp contained 3.7% sodium hydroxide during the oxygen gas treatmentstage.

(c) This pulp was bleached in the same manner as (a), but the additive(at 3% pulp concentration) was an aqueous solution containing a complexcompound of magnesium and ethylenediamine tetraacetic acid (EDTA) Thepulp contained 3.3% sodium hydroxide during the oxygen gas treatmentstage.

(d) This pulp was bleached in the same manner as (b) with the exceptionthat EDTA was added to the extracted waste liquor prior to adding to thepulp. The pulp contained 3.3% sodium hydroxide during the oxygen gastreatment stage.

(e) This pulp was bleached in the same manner as (c), with the exceptionthat diethylenetriamine pentaacetic acid was used instead of EDTA. Thepulp contained 3.3% sodium hydroxide during the oxygen gas treatmentstage.

The pulps were analyzed, and the following results were obtained:

DETPA, not EDTA.

The results for (a) show that the addition of a magnesium complex (b) to(e) greatly improves the viscosity of the pulp, when bleaching withoxygen gas. When using only diluted waste liquor as the complexingagent, as in (b), however, there is a risk that magnesium hydroxide willform, a large portion of which accompanies the pulp. This causes anincreased consumption of magnesium compound and sodium hydroxide. Theprecipitation of magnesium compounds can be totally avoided, however, byadding additional complexing agents of the type referred to (forexample, EDTA or DETPA) as in (c) to (e).Very good results are alsoobtained solely by employing complexing agents according to theinvention and (e) without using waste liquor from the process ascomplexing agent. As is evident from the foregoing the invention alsomakes possible a reduction in the amount of magnesium compound by asmuch as 66%, which is of great economic significance.

EXAMPLE 2 Birch chips about 1 mm. thick and having a lignin content of21.1% were digested in a series of four runs. In the last run, amagnesium complex prepared from MgSO and ethylenediaminetetraacetic acid(EDTA) was added.

Oxygen partial pressure, atmospheres 8 Initial temperature, C. InitialpH 13.4

Time for temperature rise from 80" to C., minutes Wood-to-liquid ratio,kg./l.

Alkali charge, NaOH kmol/ 1000 kg. dry wood, all

added at the start, before heating was begun 5 Liquor was withdrawn toobtain a wood-to-liquid ratio of 1:2.

pH after 45 minutes 11.5 Time digested at 120 C. after liquidwithdrawal,

minutes 360 pH after 405 minutes 10.8 Total digestion time, minutes 405The digestion was conducted in an autoclave rotating in a heated glycolbath. After the digestion, the chips were washed. A 63.4% yield wasobtained. The chips were dark brown in color, and the lignin contentafter the digestion was 20.4%.

(b) In a second digestion, the alkali addition was conducted in fourstages. 1.25 kmol of NaOH based on 1000 kg. of dry wood was charged inthe first stage, and in each of the three following stages, 0.75 kmol ofNaOH was charged. The digestion was otherwise carried out as before,with the exception that the alkali charge was reduced in accordance withthe aforegoing. The digestion time at each stage was 240 minutes. Thecellulose material was washed with water between each treatment stages.Subsequent to impregnating the wood with cooking liquor, the pH was 13,but fell to 10 in the liquor which was withdrawn the first time at 120C. The pH of the liquor withdrawn in the later stages was within therange from 10 to 11.

The lignin content of the pulp after the digestion was 1.5%, the pulpyield was 51.5%, and pulp brightness 75% SCAN.

(c) In another digestion, the wood was digested as in (b), but not underoxygen pressure. Burnt chips, dark brown in color, were obtained, but nopulp.

The results in (a), (b) and (c), show that if the desireddelignification effect is to be achieved at a pH of approximately 9 toapproximately 13, it is necessary that:

(i) the digestion be effected under oxygen pressure (b) (ii) the alkalicharge be added in several stages and not all ab initio (b) v. (a).

The plup obtained in (b) had a particularly high brightness.

(d) (b) was repeated, but with the addition of a magnesium complexprepared from MgSO, and ethylenediaminetetraacetic acid (EDTA). Themagnesium complex was added prior to the heating. The quantity ofmagnesium (as MgO) corresponded to 0.1% based on the dry wood. Theamount of EDTA was 0.5% based on the dry wood. In this instance, the pHvalues during the oxygen digestion were approximately 0.5 unit higherthan in Example 2b. This is related to the fact that the yield ofcarbohydrates was higher, due to the presence of the magnesium complex.The lignin content of the pulp was 2%, pulp yield was 57.5%, and thebrightness of the pulp was the same as that in Example 2b.

The results show that the yield is greatly improved by the addition ofthe magnesium complex.

Having regard to the foregoing disclosure, the following is claimed asthe inventive and patentable embodiments thereof:

1. In the process for treating cellulosic materials with alkali in thepresence of oxygen, and particularly air, the improvement whichcomprises treating the cellulose material with alkali in the presence ofoxygen and in the presence of a complex magnesium salt of anaminopolycarboxylic acid or alkali metal salt thereof, the complexmagnesium salt reducing or entirely preventing attack of oxygen on thehemicellulose and cellulose in the course of the process.

2. A process according to claim 1, in which the aminopolycarboxylic acidhas the formula:

wherein A is selected from the group consisting of -CH COOM and --CH CHOH, M is hydrogen or an alkali metal, and n is a number within the rangefrom zero to five.

3. A process according to claim 2, wherein the complex magnesium salt isa chelate of magnesium and a nitrilotriacetic acid.

4. A process. according to claim 2, wherein the complex magnesium saltis a chelate of magnesium and an ethylenediaminetetraacetic acid.

5. A process according to claim 1, wherein the treatment is carried outon lignin-bearing wood cellulose to remove lignin at least in part.

6. A process according to claim 1, including a complex magnesium salt ofa magnesium-complexing organic acid contained in waste liquor from aprocess in which cellulosic material is treated with alkali.

7. A process according to claim 6, wherein a magnesium salt is added tothe waste liquor to form the complex magnesium salt in situ in theliquor.

8. A process according to claim 1 in which the quantity ofaminopolycarboxylic acid is sufiicient to solubilize substantially allof the magnesium present as complex magnesium salt.

9. A process according to claim 1 in which the amount of magnesiumcomplex is within the range from about 0.005 to about 0.5%, calculatedas MgO and based on the weight of the cellulosic material.

10. A process according to claim 1 in which the mole ratio ofaminopolycarboxylic acid per mole of magnesium is within the range fromabout 0.01:1 to about 15:1.

-11. A process according to claim 1, wherein the cellulosic material isan unbleached, partially bleached or bleached cellulose sulfate pulp,sufite pulp or semichemical pulp derived from wood.

12. A process according to claim 1 wherein the cellulosic material iscellulose pulp and the partial pressure of the oxygen at the beginningof the treatment is at least about 1 atm.

13. A process according to claim 1 wherein the cellulosic material iscellulose pulp and the treatment is carried out at a temperature withinthe range from about 80 C. to about 130 C.

14. A process according to claim 1 wherein the cellulosic material iscellulose pulp and the treatment is carried out at a concentration ofcellulosic material in excess of up to about 35% 15. A process accordingto claim 1 wherein the cellulosic material is cellulose pulp and thecellulosie pulp prior to being treated is impregnated with an aqueoussolution of the complex magnesium salt or components which in thesolution form a complex magnesium salt.

16. A process according to claim 15, wherein a portion of the solutionis removed from the pulp prior to the treatment.

17. A process according to claim 1 wherein the cellulosic material iscellulose pulp and the quantity of alkali calculated as NaOH is withinthe range from about 0.5 to about 10% based on the dry weight of thecellulosic material.

18. A process according to claim 1, wherein the source of magnesium isMgSO MgO, MgCl Mg(OH) MgCO or Mg(NO 19. A process according to claim 1,wherein the complex magnesium salt includes in addition a complexmagnesium salt of an aliphatic aor fi-hydroxycarboxylic acid.

20. A process according to claim 19, wherein the hydroxycarboxylic acidis an aliphatic hydroxy acid having from two to about twelve carbonatoms and from one to about ten hydroxyl groups.

21. A process according to claim 1 in which the cellulosic material iswood.

22. A process according to claim 21 in which the wood is hardwood.

23. A process according to claim 21 in which the wood is softwood.

24. A process according to claim 21 in which the wood is in particulateform.

25. A process according to claim 21 in which the amount of alkali at thebeginning of the digestion is limited to at most 75% of the total molarquantity of alkali required for the digestion, and the remainder of therequired alkali is added progressively during the digestion, whilemaintaining the pH of the digestion liquor in the course of thedigestion within the range from about 9.2 to about 13 during the firststages and the major portion of the digestion process.

26. The process of claim 25, in which the amount of alkali is Within therange from about 5 to about 20% of the total molar quantity of alkalirequired.

27. The process of claim 25, in which the alkali is added continuouslyduring the digestion.

28. The process of claim 25, in which the alkali is added in incrementsduring the digestion.

29. The process of claim 25 in which the pH is within 7 the range fromabout 9.5 to about 11.5.

30. The process of claim 25, in which the pH is permitted to drop below9, down to about 8, during the final stages of the process withoutseriously alfecting the quality of the pulp.

31. The process of claim 25 in which the total amount of alkali iswithin the range from about 1 to about 10 kilomoles per 1000 kg. of drywood.

32. The process of claim 31 in which the cellulose pulp produced is aviscose, acetate or cuprammonium pulp, and the amount of alkali iswithin the range from about 6 to about 8 kilomoles per 1000 kg. of drywood.

33. The process of claim 31 in which the cellulose pulp produced is asemichemical pulp, and the amount of alkali is within the range fromabout 1 to about 2 kilomoles per 1000 kg. of dry wood.

34. The process of claim 31 in which the cellulose pulp produced is apaper pulp and the amount of alkali is within the range from about 2.5to about 5 kilomoles per 1000 kg. of dry wood.

35. The process of claim 31 in which the cellulose pulp produced is apulp for use in making fine paper, plastic fillers, and soft paper ortissue paper, and the amount of alkali is within the range from about 2to about 6 kilomoles per 1000 kg. of dry wood.

36. The process of claim 1 in which the alkali is any alkali metalhydroxide or alkali metal carbonate.

37. The process of claim 36 in which the alkali metal hydroxide issodium hydroxide.

17 18 38. The process of claim 36 in which the alkali metal ReferencesCited carbonate is sodium carbonate.

39. The process of claim 36 in which the alkali is a UNITED STATESPATENTS mixture of alkali metal hydroxide or alkali metal car- 36523853/1972 Noreus et a1 16265 X bonate with an alkali metal bicarbonate. 53,652,386 3/1972 Noreus et 162 65 40. The process of claim 39 in whichthe alkali metal bicarbonate is sodium bicarbonate which serves as aLEON BASHORE Primary Examiner buffer. A. L. CORBIN, Assistant Examiner41. The process of claim 39 in which the amount of alkali metalbicarbonate is within the range from about 1O 1 to about 5 kilomoles per1000 kg. of dry wood. 3 111; 1 2. 72 75

